1. Field of the Invention
The present invention relates generally to an automotive suspension system having adjustable suspension characteristics depending upon vehicle driving condition. More specifically, the invention relates to an actively controlled automotive suspension system which varies suspension characteristics in a positive manner for suppressing attitude change of the vehicle. Further particularly, the invention relates to an anti-rolling and/or anti-pitching suspension control which can avoid self-induced vibration which may otherwise occur.
2. Description of the Background Art
The U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987, to Hirotsugu YAMAGUCHI et al., and assigned to the common assignee to the present invention, discloses an actively or positively controlled automotive suspension system which includes hydraulic cylinder for generating variable damping force against relative motion of a vehicular suspension member and a vehicle body in bounding and rebounding directions for adjusting suspension characteristics and adjusting vehicle body attitude.
Similar types of actively controlled suspension systems have been disclosed in the co-pending U.S. patent application Ser. No. 060,911, filed on June 12, 1987 and assigned to the common assignee to the present invention. Further systems are described in European Patent First Publications Nos. 02 49 246 and 02 49 209 respectively corresponding to co-pending U.S. patent applications Ser. Nos. 061,368 (filed on June 15, 1987) and 059,888, filed on June 9, 1987, both of which have been assigned to the common assignee to the present invention. The later-mentioned three inventions includes features of anti-rolling and/or anti-pitching suspension control for adjusting damping characteristics at respective vehicular wheels for suppressing vehicular rolling motion.
Though such prior proposed actively controlled suspension systems are successful in suppressing rolling motion and/or pitching motion caused in the vehicle body, at a certain level, a problem can be encountered in generating self-induced vehicular body vibration in rolling and/or pitching directions. Namely, in the actively controlled suspension system, suspension characteristics are controlled depending upon the vehicle driving parameters which are representative of vehicular rolling and/or pitching motions. When the vehicle attitude changes in rolling or pitching directions, the inertial moment generated in the vehicle body by the rolling or pitching motion tends to be included in the rolling and/or pitching magnitude indicative parameter. This causes over-shooting to generate self-induced vibration. The self-induced vibration may degrade riding comfort and drivability of the vehicle.